In general, a large-sized transport vehicle called a dump truck is provided with a vessel (loading platform) on a frame of a vehicle body tiltably in an upper-lower direction on a basis of the rear side. The transport vehicle carries and transports objects (for example, crushed stones or earth and sand) to be transported to a transportation destination (cargo unloading site or cargo collection site) in a state where the objects to be transported are loaded in a large amount on the vessel (Patent Document 1).
A transport vehicle of this type according to the conventional art comprises schematically an automotive vehicle body, a vessel which is provided on the vehicle body tiltably (liftably) in an upper-lower direction on a basis of the rear side and on which the objects to be transported are loaded, a hoist cylinder which is provided between the vessel and the vehicle body and tilts the vessel upward or downward with expansion or contraction of a rod therein, a hydraulic source comprising a hydraulic oil tank for reserving hydraulic oil and a hydraulic pump for delivering pressurized oil as the hydraulic oil to supply the pressurized oil to a bottom-side oil chamber or a rod-side oil chamber in the hoist cylinder for expanding or contracting the hoist cylinder, and a control valve device provided between the hydraulic source and the hoist cylinder to control supply and discharge of the pressurized oil to and from the bottom-side oil chamber or rod-side oil chamber in the hoist cylinder.
The control valve device used in the transport vehicle has a total of four switching positions composed of a raising position of supplying the pressurized oil from the hydraulic pump to the bottom-side oil chamber and discharging the hydraulic oil in the rod-side oil chamber to the hydraulic oil tank to expand the hoist cylinder for upward tilt (rotation) of the vessel, a lowering position of supplying the pressurized oil to the rod-side oil chamber and discharging the hydraulic oil in the bottom-side oil chamber to contract the hoist cylinder and lower down (downward rotate) the vessel, a floating position of discharging the hydraulic oil in the bottom-side oil chamber by self-weight of the side of the vessel to contract the hoist cylinder and allow for self-weight fall of the vessel, and a neutral position of stopping supply and discharge of the pressurized oil to and the from the hoist cylinder to stop movement of the hoist cylinder for holding the vessel.
Here, the control valve device is selectively switched to any switching position of the total of the four switching positions in response to a manual operation of an operating lever which an operator operates. The transport vehicle self-travels to the transportation destination in a state of loading the objects in the vessel and thereafter, expands the hoist cylinder to diagonally backward rotate the vessel. This raising operation allows the objects to be dumped to a cargo unloading site along the tilting direction of the vessel.
That is, when the operation of the operating lever by the operator switches the control valve device from the neutral position to the raising position, the pressurized oil from the hydraulic pump is supplied toward the bottom-side oil chamber in the hoist cylinder and the hydraulic oil in the rod-side oil chamber therein is discharged to the hydraulic oil tank. In consequence, the hoist cylinder expands to rotate the vessel in such a manner as to be greatly tilted to the vehicle body backward side, thus discharging (dumping) the loads in the vessel outside thereof in a sliding/falling manner.
On the other hand, after the dumping operation of the objects is completed, when the control valve device is switched from the raising position to, for example, the floating position by the operation of the operating lever, the hydraulic oil in the bottom-side oil chamber of the hoist cylinder is discharged by the self-weight of the side of the vessel to contract the hoist cylinder. Therefore, the vessel gradually goes down by the self-weight to a position of being seated on the vehicle body.
At traveling of the vehicle, for example, the operating lever is held to the floating position. Thereby, the vessel continues to be seated on the vehicle body by the self-weight, and the hoist cylinder also can be held in the contraction state by using the self-weight of the side of the vessel.
Incidentally, vehicle traveling paths of a crushed stone site or the like in a mine are dirt roads, and mostly have ragged road surfaces. In a transport vehicle traveling on such a road, vibrations caused by the bumpy road surface at traveling become larger in magnitude. At this time, in a case where the vessel is empty in loading, the vessel floats up from the vehicle body following vibrations caused by knocking-up from the road surface, and thereafter, the vessel collides with the vehicle body at the time when the vessel is again seated on the vehicle body. Therefore, uncomfortable feelings are given to the operator in the cab or repeated collisions of the vessel with the vehicle body degrade the durability and lifetime.
Therefore, in the transport vehicle according to Patent Document 1, in a case where the vehicle body is in the traveling state and the control valve device is in the floating position, when it is detected that the vessel moves in a direction of floating from the vehicle body, the control valve device is controlled from the floating position to the lowering position by a control unit. Thereby, when the vessel tends to float from the vehicle body, the control valve device is switched from the floating position to the lowering position, and thereby, the vessel can be pressed to the vehicle body side to restrict the floating-up of the vessel.